Polarizing modulator for an electronic stereoscopic display

ABSTRACT

A polarizing modulator for use in an electronic stereoscopic display system having a sequentially scanning display includes a plurality of liquid crystal segments arranged contiguously in a direction of the sequential scan. The liquid crystal material used in each segment has its phase shift tuned to eliminate the perception of a visible line between segments. In a preferred embodiment, the phase shift is tuned by applying a bias voltage to the liquid crystal in its low state.

FIELD OF THE INVENTION

The invention is a liquid crystal (“LC”) modulator mounted in front of acathode ray tube (“CRT”) or similar display screen, which switchesbetween left- and right-handed states of circular polarization todisplay stereoscopic images which are viewed with passive analyzingeyewear. The invention uses a horizontally segmented or Byatt modulatorto suppress phosphor afterglow crosstalk, and a unique electronic drivescheme to eliminate the appearance of the individual segments.

BACKGROUND OF THE INVENTION

The seminal disclosure in the field is U.S. Pat. No. 4,281,341 by Byatt.This patent describes the use of a switchable polarizer in combinationwith a television monitor to produce images which are alternatelyvertically or horizontally polarized. The switchable polarizer is an LCcell of the twisted nematic type. The most important art disclosed isdiscussed beginning at column 3, line 26:

-   -   Because liquid crystal cells switch between their two        polarization states relatively slowly, it may be desirable to        divide each cell into two halves, one half corresponding to the        top half of a television picture, the other corresponding to the        lower half, so that when the top half of the television raster        pattern has been scanned the top half of the liquid crystal cell        can be switched into its next required polarization state, so        that it has settled into this state by the time that the top        half of the raster is required to be scanned on the next frame        period. Similarly, the bottom half of the liquid crystal cell        would be switched, whilst the top half of the raster pattern is        being scanned.

It is this suggestion by Byatt that has made it possible to create anon-screen modulator with desirable performance characteristics.Specifically, Tektronix and its successor NuVision have for a number ofyears manufactured devices following Byatt's suggestion. In one of thelatest versions, five segments are used. These segments are animated, orscanned, in synchronization with the electron beam as the rasterprogresses from the top to the bottom of the display screen. Theprincipal benefit of this approach is to suppress crosstalk occurringfrom the phosphor afterglow.

In a timed-multiplex stereoscopic display there are two componentsresponsible for producing crosstalk. One factor is the incompleteocclusion of the shutter, and the other is the afterglow of phosphorsinto the immediately adjacent field. In addition to the term crosstalk,which implies objective measurement, the terms leakage and ghosting arealso used. The term leakage also implies a value derived by measurement.The term ghosting implies an observable but subjective entity.

When products such as CrystalEyes eyewear, manufactured byStereoGraphics Corporation, are used, there is no opportunity tosuppress the afterglow component of the alternate field stereo-visiondisplay because LC shutters are used in front of the eyes. Theseshutters are shuttering out of phase with each other but in synchronywith the video field rate, so that each eye sees only its requiredimage. In this case, if the dynamic range (the ratio of the transmissionof the shutter in its open state to the transmission of the shutter inits closed state) of the shutter is sufficiently high, there is littleleakage of light through the shutter when it is occluded. However, theeye which is seeing through the open shutter is also seeing a faint orghostly image of the prior field due to the long decay characteristicsof the phosphor set used in monitors for television and computergraphics. In particular, tile green phosphor has the longest visibletail, something which can be easily demonstrated by turning off theappropriate monitor electron gun.

CrystalEyes eyewear uses a high-dynamic-range shutter (typically betterthan 500:1). However, as already mentioned, the CrystalEyes approachcannot squelch phosphor afterglow. On the other hand, the Byattmodulator is able to do so (as will be explained), but the particular LCdevice that Byatt suggested, the twisted nematic, has proved to be lessuseful in this application than the surface-mode device, or π-cell, asit is more commonly called.

There is a body of literature that describes the functioning of theπ-cell, and we cite some of it: U.S. Pat. No. 4,884,876 (Lipton et al.);U.S. Pat. No. 4,719,507 (Bos); and U.S. Pat. No. 4,566,758 (Bos). Thefollowing are references discussing using a π-cell in the form of alarge modulator for field switching: “High-Performance 3-D ViewingSystems Using Passive Glasses” by Bos et al. (p. 450, SID '88 Digest);and “Field-Sequential Stereoscopic Viewing Systems Using PassiveGlasses” by Haven (Proceedings of the SID, vol. 30/1, 1989). Inaddition, Johnson and Bos, in their article “Stereoscopic DisplayPerformance” (ELECTRONIC IMAGING EAST CONFERENCE, Building ApplicationSolutions with Today's Imaging Tools, 1990), describe in detail how theByatt shutter improves performance in terms of suppression of ghostingcreated by phosphor afterglow. Because of the existence of this priorart literature, there is little reason to go into the explanation of thephysics of the device in great detail.

The multiple-segment Byatt modulator has a noticeable drawback: thesegments are visible as individual units, especially when the imagecontains light-colored neutral backgrounds. Thus, whenever there istexture or an image complexity, the segments are more difficult to see.

The LC cell used for the Byatt device has an LC gap (material thickness)of typically five or six microns. Such cells are coated with aconductor, such as indium tin oxide (“ITO”) on the inside surfaces. If athin line is scribed away from the ITO, leaving dielectric instead ofconductor, the electrical continuity is broken and separate electrodesegments are produced. Typically, only one of the two facing ITOcoatings needs to be so scribed. The scribing can be very thin; so thin,in fact, that hopefully it cannot be seen. We have made parts where thedielectric scribe is 25 microns.

We have established that the scribe is actually not visible, but rather,the source of the segmentation artifact is the change in the shading, ordensity and coloration, at the boundary line between each segment. Thesource of this shading is understood and described in the literaturecited above. What the observer sees is the density and color changedifference between the segments, and each segment appears to stand outas a visible entity in contrast to the immediately adjacent segment. Theimpression one gets (and it is an optical illusion) is that a thin lineseparates the segments. The natural conclusion is that the scribed linein the conductor is visible, and that the thinner the scribe, the lessvisible it will be. However, this is not so. We have produced a scribedline which is five times thicker than the scribed line of the NuVisionproduct, but our segments are invisible, while theirs are visible.

This ability to distinguish segments is a distracting visual artifact.Indeed, this selection device (i.e. modulator plus eyewear) is typicallyused in high-end applications for scientific visualization or for workin aerospace and the military. The users of such devices do not want tobe distracted by the visibility of the individual segments. In fact, itis the principal complaint lodged against this device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principal components used for on-screen electro-opticalswitching of polarization for image selection in a stereoscopic display.

FIG. 2 illustrates the visibility of the Byatt shutter segments in theprior art.

FIG. 3 illustrates the waveform used to drive prior-art parts,incorporating both a carrier and zero-voltage bias.

FIG. 4 illustrates the invisibility of the individual segments of thepresent invention.

FIG. 5 is a drawing of the waveform of the drive signal used for thepresent invention.

FIG. 6 shows the “animation” sequence for the Byatt multi-segmentedshutter.

FIG. 7 illustrates the directors within an LC cell in two differentstates.

FIG. 8 is a block diagram of the LC driver electronics of the presentinvention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows the major components used in the present invention. Thedisplay monitor 101 produces an image onto CRT screen 102 in aconventional manner. The light produced by the CRT display 102 istransmitted through circular sheet polarizer 103 and Byatt segmented LCmodulator 104 (segments not shown), oriented such that the light emittedby the CRT display is circularly polarized. Image origination device106, which may be a computer, produces images which are transmitted viacable 105 for display by monitor 101. Video field synchronizationinformation is conveyed from the image origination device 106 via cable107 to electronics driver 108. The electronics driver 108 produces thewaveform shown in FIG. 5. Driver 108 is used to drive the Byattsegmented modulator 104 via cable 109. The Byatt segmented modulator 104is shown in more detail in FIG. 4.

The image is viewed with passive circular polarizing eyewear 112 withleft-handed and right-handed circular polarizers 110 and 111,respectively. Those familiar with the art will understand that thehandedness of the circular polarizers may be interchanged and that themodulator 104 may have its polarization characteristics controlled bythe phase of the drive signal of driver 108 (see FIG. 5), or byselecting either a left-handed or right-handed circular polarizer forpart 103.

When a video field is produced by electronic imaging device 106, it iswritten on the CRT display screen 102. Video or electronic displaysignals are made up of a succession of fields and vertical blankingintervals with synchronization pulses. The synchronization pulses aresensed by driver 108 and are used for the synchronization of the signalnecessary to drive the segmented shutter 104 in synchrony with thelocation of the scanning electron beam. The segments of the Byattshutter 104 are “animated” to follow the beam as it writes on the faceof display screen 102 from top to bottom.

In an electronic stereoscopic display that runs at 120 fields per second(a good value for elimination of flicker), the duration of a field isapproximately eight milliseconds, so it takes eight milliseconds for thebeam to start at the top and scan to the bottom of the screen. Thesegments of the Byatt shutter 104 are driven in synchrony with the beamso they follow the beam and are actually switched in synchrony with thelocation of the beam. A more complete description of how the Byattsegmented shutter 104 works will be given in conjunction with theexplanation of FIG. 6, but first, the groundwork for a full appreciationof the explanation must be provided.

FIG. 2 is an illustration of the visibility of segments in the prior artmodulator. Modulator 201 has, for the purposes of illustration, fivesegments labeled 202 through 206. Each segment has a boundary between itand the adjacent segment, and these boundaries, which are horizontallines, are labeled 207 through 210. Shading has been added at theboundaries 207 through 210 to indicate that the individual segments ofthe prior art device are visible.

We have illustrated the shading effect for one eye only, in particularfor the right eye view, given our particular configuration of polarizer,analyzer, and phase of the drive voltage. The shading is as follows: atthe boundary of the scribed line, the area of the top segment adjacentto the line shades to a lighter tone, and the area of the lower segmentimmediately below the boundary is darker and becomes lighter. The othereye, the left eye, has a reversed pattern (not illustrated), in whichthe top segment shades to a darker area immediately adjacent to theboundary scribe, and the lower segment is lighter in tonality andbecomes darker to match the tonality of the entire segment. The effectis visible for just a few millimeters on either side of the boundaryline.

FIG. 3 is a representation of the drive waveform which is produced byprior art electronics. For example, in FIG. 1 these electronics arecontained within part 108. We see that a carrier is used to modulate thewaveform 301, and typically a 2 KHz carrier is employed. In this case,the carrier is driven to plus or minus H volts, where H is typically 15to 20 volts. Also, in the regions 302 between drive voltage H, thevoltage is zero. Or, to use the nomenclature we have chosen here, thebias is zero volts. Observe that the periods for applying voltage andbias are of equal duration.

FIG. 4 is an illustration of the Byatt modulator 401 in accord with thepresent invention, with five segments labeled 402 through 406. Thedotted lines between the segments illustrate that the segments arepresent but impossible to see, and indeed the shutter appears to be asingle integral segment in its entirety.

FIG. 5 is a drawing of the waveform used to drive Byatt modulator 401,and FIG. 8 (discussed below) is a block diagram of the circuit used toproduce the waveform. As shown in FIG. 5, the waveform includes portion501 which has a positive voltage of value +H and portion 503 which has anegative voltage of value −H. Thus, the device is driven between +H and−H volts (typically between 15 and 20 volts). For example, if we drivethe shutter at 40 volts peak-to-peak, +H is 20 volts and −H is −20volts. Each quarter cycle of the waveform has a duration T and eachquarter cycle interval is signified by the designations A, B, C, D. Themodulator 401 is driven to plus or minus H volts for equal durations T.Waveform portions 502 and 504 are defined as the bias voltage. Theseintervals B and D are of the same duration T as intervals A and C. Thebias voltage for intervals B and D have a value of plus and minus Lvolts.

There are two major differences between the prior art waveform shown inFIG. 3 and the waveform used in the present invention as shown in FIG.5. In the present device, there is no carrier. Instead, a bias voltageis used. The lack of carrier modulation results in a lower powerrequirement, and this has been described in U.S. Pat. No. 4,884,876entitled Achromatic Liquid Crystal Shutter for Stereoscopic and OtherApplications, by Lipton et al.

It is the application of the appropriate bias voltage L, as shown inFIG. 5, that eliminates the visibility of the individual electrodesegments. The segments are visible at L=0 volts, as illustrated in FIG.2, but become invisible with the application of the proper value of biasvoltage, as shown in FIG. 4. The application of a bias voltage to amodulator driven with a carrier, as shown in FIG. 3, has the sameresult, namely the segments disappear as individual entities. In thiscase the bias voltage is also modulated by the carrier.

We have used a five-segmented shutter having a 5.2 micron LC gap filledwith liquid crystal material, such as Merck ZLI-1565. The shutter wasdriven at plus or minus 18 volts. With a bias voltage of zero volts, theindividual segments were visible. However, when a bias voltage of plusor minus 1 volt was applied, the segments as individual entitiesentirely disappeared; that is, the segments could not be distinguishedfrom one another. This astounding and unexpected result held for biasvalues up to plus and minus 2 volts. The ability to make the segmentsappear to be integral, as if they are one single segment, is thedifference between a modulator which is merely serviceable, as in theprior art, and one which is excellent, such as the present invention.

The value of the required bias is a function of the value of drivevoltage. If the drive voltage is increased, then the bias mustaccordingly be increased to achieve the beneficial result. Measurementshave showed that the speed of the device, from low voltage to highvoltage and vice versa, was unaffected with the application of the bias.Transmission and dynamic range as measured with a photometer weresimilarly more or less constant. Thus, there was no diminution inperformance as a result of the application of the bias.

With reference to FIG. 6, we will describe how the Byatt modulatorachieves the desired crosstalk reduction. At time T=1, the electron beam(R BEAM) has written the first lines of the right image in segment 1(segment numbers are given in a column on the left edges of thedrawings). The vertical blanking, as noted on the drawing labeled T=1,immediately precedes the first line of the right image. The beamcompletes writing the right image R in the area of segment 1, in whichcase segment 1 is in one of two possible states: it is either drivenwith drive voltage plus or minus H or bias voltage plus or minus L.

For didactic simplification, we will assume segment 1 is being driven atdrive voltage H, and we will label this “state 1.” In the meantime,segments 2, 3, 4, and 5 are in state 2 (driven at bias voltage L) andcontinue to show the phosphor afterglow of the previously written leftfield. At time T=2, both segments 1 and 2 are in state 1, whereassegments 3, 4, and 5 are in state 2. The reader will be able to see bylooking at the drawings for T=3, T=4, T=5 and T=6 that similardescriptions can be given but will be omitted here. At T=5, all fivesegments are showing the right image, and at T=6 the left beam hasstarted to be written in segment 1. At T=6, segment 1 has switched tostate 2, and segments 2 through 5, are now in state 1. The cyclecontinues, and R and L segments are shunted to the appropriate eyebecause the observer is viewing the image through a selection devicemade up of left and right handed circular polarizer analyzers.

When viewing images with the technique, as opposed to that used inCrystalEyes or in other shuttering eyewear approaches, both eyes arealways seeing an image. The eyes are not alternately occluded; the righteye is seeing a right image as the left eye is seeing a left image. Thisis not true in shuttering eyewear because the eyes are seeing images outof phase. The important thing here is that the afterglow component ofthe phosphor-emitted light is transmitted to the appropriate eye. Inother words, the left image continues to go to the left eye instead ofinterfering or mixing with the right eye image and showing up as aghost-like double exposure. The segmented shutter is thus able topresent a vastly improved image by animating the segments in synchronywith the beam location, thereby suppressing the afterglow componentwhich produces ghosting because the afterglow component has beentransformed into an image for the appropriate eye.

A good stereoscopic image results despite the fact that, as measuredphotometrically, this modulator and analyzing eyewear (which togetherform a shutter) have a relatively low dynamic range. The dynamic rangeis quite a bit less than the dynamic range one measures with CrystalEyesshutters. Clearly, much of the crosstalk one sees in such a system mustoriginate from phosphor afterglow.

We have described how the segmentation approach reduces crosstalkbetween left and right eyes. In other words, a segmented shutter is ableto suppress the ghost image so that one can see an image which isrelatively unencumbered by the artifact. Having created such a benefit,it is a pity that the segments should continue to be visible asindividual entities.

Observers assume that they are seeing lines between the segments, whenactually the problem is color and density shading within each segment.The abrupt transition from segment to segment creates an opticalillusion and seems to define a sharp horizontal line, and as mentionedabove, this misperception of the problem lead prior workers in anon-productive direction, namely placing an inordinate emphasis on thereduction of the scribe width between segment electrodes. That is not tosay that a thin scribe is not important, because obviously, a widescribe will be visible even if the segment shading suppression techniquedescribed herein is applied.

The following may help to explain what occurs within the cell as biasvoltage is applied. With reference to FIG. 7, a π-cell is shown in twostates, namely state 701 and state 702. State 701 exists when the n-cellhas maximum voltage applied, and state 702 exists when the minimumvoltage is applied. This minimum voltage may be zero volts or the biasvoltage L, as described above. Elements 703, 704, 705 and 706 refer tothe glass walls of the π-cells including the interior ITO electrodecoatings (now shown) and director alignment layer coating (not shown).Elements 707, 708, 710 and 712 refer to the directors immediatelyadjacent to the director alignment layer. The directors are shownthroughout the two diagrams as dash-like lines. The director alignmentis usually made of polyimide overcoating the ITO layer, which is rubbedor buffed to produce micro-abrasions. The directors (ordered groups ofLC molecules) line up according to the rub suggested in the polyimidelayer. It is assumed that polarizers are employed on both outsidesurfaces of the glass walls. The polarizers are aligned with their axescrossed and oriented at 45° to the surface directors.

As previously mentioned, state 701 is the high voltage state and state702 is the low voltage state. (There is an additional π-cell state inwhich voltage H has not been applied for some considerable time, saytens of milliseconds. This is the relaxed state and does not concern ushere.) The bulk of the LC fluid is shown within brackets labeled 709 and713. With reference to state 701, the high voltage state, the directorsin the bulk 709 are dipoles whose major axes are aligned with theelectric field (not shown). The lines of force are perpendicular to thesurface of the glass walls 703 and 704, and that is the orientationfollowed by the major axes of the bulk directors.

In state 702, the low voltage or bias voltage state, tile directors ofthe bulk 713 are splayed and lined up to a greater extent with thetipped directors at the surface. In the high voltage state 701, there isno phase shift, because there is little opportunity for the surfacedirectors to produce retardation, but in the case of the low voltagestate 702, the orientation of the splayed directors in the bulk 713adjacent to the surface produces sufficient retardation to toggle theaxis of transmitted incoming linear polarized light. A similar eventoccurs in the case of circularly polarized light, but in this case thehandedness of the circularly polarized light is reversed.

The application of bias to a surface mode part tunes the phase shift λof the device, but the phase shift can similarly be tuned by adjustingthe thickness of the gap d, or by use of an LC material with a differentbirefringence Δn. The phase shift is given by the relationship:λ=Δnd.

When λ=π radians, the axis of linearly polarized light is rotatedthrough 90° and maximum extinction of transmitted light will occur. Thephase shift λ may be tuned, if desired, by selecting an LC material withthe appropriate Δn, by adjusting the thickness of the film of LCmaterial, gap d, or by applying the proper voltage as described above.In the case of adjusting the bias voltage, the degree of splayedness ofthe directors in the bulk 713 can be controlled. Clearly, the greaterthe bias voltage, the more the directors will be aligned as parallel tothe applied electric field as they would be in the on state. Thus it ispossible to tune the retardation or phase shift of the cell in the lowvoltage state by varying the bias voltage.

What is happening to achieve the reduction of shading within eachsegment, and how does one tune the phase shift? Unfortunately, thelinkage between the application of the bias and the disappearance of thesegments is obscure. Surely the mechanism is related to the change indirector orientation and the amount of phase shift. It is probable thatthe desired result can also be achieved by changing the phase shiftrelated parameters given above. While it is true that we haveestablished that the desired result is produced by the application ofbias, varying the other parameters appropriately might also eliminatethe segments as individual visible entities. However, only theapplication of bias can be smoothly and continuously varied, unlike theother parameters which must be varied discretely, and would also requirebuilding individual cells with incrementally varying parameters.

With regard to the tuning of the device, the best way to do so isempirically. A test target of continuous textureless background isdisplayed on the CRT, and the bias voltage is varied. Once the visiblesegments of the Byatt modulator are eliminated, the desired result hasbeen achieved.

With reference to FIG. 8, the drive electronics circuit is illustrated.The circuit receives as its input a Left/Right drive signal that is highwhen a left eye image is visible and low when a right eye image isvisible. This signal is processed by a single-chip microcomputer (MCU)801, such as a Motorola MC68HC05. The input signal switches coincidentwith tile vertical sync pulse. Normally this is at or very near thebeginning of the vertical blanking interval. After the blanking intervalcomes the active video, and the pattern repeats.

The MCU 801 is interrupted by edges of the input signal. Using theon-chip timing resources, the MCU measures the time between these edges.The accuracy of this timing is a function of the frequency of crystal802, in this case 8 MHz, which results in a basing time-keeping accuracyof 1 μsec. The MCU is thus executing a software Phase-Locked-Loop (PLL).

Once the internal timing is established, the MCU 801 uses thisinformation to create the appropriate transition points for eachsegment. First, the field time is calculated. This is the length of timebetween transitions of the input signal. Second, the blanking time iscalculated at 1/16 of the field time. This value is an acceptableapproximation for all resolutions and display modes in common use. Next,the segment time is calculated at three times the blanking time or 3/16of the field time. The total field is 1/16 blanking plus five time 3/16segments.

In this case, each segment should be driven to its proper stateapproximately 2 msec before the beam sweeps past the beginning of thedisplayed segment area. This selected value of 2 msec is a function ofthe optical transition speed of the LC polarizer. Thus, the firstsegment must switch at 2 msec minus the blanking time before the inputsignal edge. Likewise, the second segment switches 3/16 of the fieldtime later, and so on.

Transmigration is a damaging deterioration of the cell which occurs whenthe net average DC level applied across the cell is not zero. For eachmsec that the cell has a voltage of positive H volts applied across it,there must be a msec where the cell has negative H applied across it.This also applies to the low bias voltage. In the waveform shown in FIG.5, regions 501 and 504 are positive voltages and regions 502 and 503 arenegative voltages. Four fields are required before the waveform appliedto the cell repeats.

The MCU 801 outputs two status bits per segment, namely an on/off bitand a polarity bit. Each segment has a driver circuit consisting of a4:1 analog multiplexor 803, an amplifier 804, and a filter 805. The 4:1MUX 803 takes the two status bits and routes one of four analog voltagesinto the amplifier 804.

Normal operating voltages for the LC polarizer are in the area of 40volts peak-to-peak. Thus, the high and low operating voltages theamplifier 804 is required to deliver to the cell are +20 and −20 volts.The MUX 803 would have to switch these voltages. However, if theamplifier is given a gain of −10, then the MUX only needs to switchvoltages of +2 and −2 Volts. This allows the use of a much lessexpensive multiplexor while having a tiny increase in the cost of theamplifier circuit. In this case, the amplifier gain is −10 and the fourvoltages switched by the MUX are: −1.8V (corresponding to 501 in FIG.5); +100 mV (corresponding to 502 in FIG. 5); +1.8V (corresponding to503 in FIG. 5); and −100 mV (corresponding to 504 in FIG. 5).

The output of the amplifier is filtered before reaching the LCpolarizing panel. Low-pass filters 805 are used to suppress emissionsfor regulatory certification purposes rather than to have an effect onthe LC polarizing panel.

Status indicators 806 are controlled by MCU 801 to indicate the statusof the unit (i.e. power on, input signal detected, etc.) and, whenflashing, to indicate errors (i.e. input unstable, duty cycle not 50%,frequency out of range, etc.).

We have established that by applying the proper bias voltage value, orpossibly by tuning the retardation or phase shift of the cell by any oneof several means as specified above, the shading of individual segmentsis entirely eliminated, and thus the individual segments cannot be seen.Thus, the benefit of the Byatt shutter, in terms of its ability tosuppress ghosting may be fully enjoyed, while the visibility of theindividual segments is entirely suppressed.

1. In an electronic stereoscopic display system of the type whereinimages are sequentially scanned across an electronic display, whereinthe display, a polarizing sheet and a modulator are held injuxtaposition and observed by a user with polarizing eyewear, whereinthe modulator includes a plurality of segments each having liquidcrystal material and arranged contiguously in a direction of thesequential scan, and wherein each segment is selectively driven incorrespondence with the scan by applying and removing a drive voltage todrive the liquid crystal to a high state and a low state, respectively,wherein the improvement comprises tuning the phase shift of each liquidcrystal to eliminate the perception of visible lines between thesegments.
 2. An electronic stereoscopic display system as in claim 1,wherein the phase shift is tuned by applying a bias voltage to drive theliquid crystal in its low state.
 3. An electronic stereoscopic displaysystem as in claim 1, wherein the phase shift is tuned by selecting aliquid crystal material having a different birefringence.
 4. Anelectronic stereoscopic display system as in claim 1, wherein the phaseshift is tuned by adjusting a gap thickness of the liquid crystalmaterial.
 5. An electronic stereoscopic display system as in claim 2,wherein each segment is driven with the drive voltage to its high statein synchrony with the scan for a selected eye.
 6. An electronicstereoscopic display system as in claim 5, wherein each segment isdriven with the bias voltage to its low state in synchrony with the scanfor a non-selected eye.
 7. An electronic stereoscopic display system asin claim 1, wherein the bias voltage is a proportional fraction of thedrive voltage.
 8. A polarizing modulator for an electronic stereoscopicdisplay system having a sequentially scanning display, comprising: aplurality of segments each containing liquid crystal material andarranged contiguously in a direction of the sequential scan, and drivingcircuitry coupled to each segment and adapted to drive each segment to ahigh state with a drive voltage and to a low state with a bias voltagesuch that an optical artifact creating a perception of visible linesbetween the segments is eliminated.
 9. A polarizing modulator as inclaim 8, wherein each segment is driven with the drive voltage to itshigh state in synchrony with the scan for a selected eye and whereineach segment is driven with the bias voltage to its low state insynchrony with the scan for a non-selected eye.
 10. An electronicstereoscopic display system as in claim 8, wherein the bias voltage is aproportional fraction of the drive voltage.
 11. An electronicstereoscopic display system as in claim 8, further comprising a circuitadapted for varying the bias voltage.
 12. A drive circuit for amodulator in an electronic stereoscopic display system having asequentially scanning display, wherein the modulator includes aplurality of segments each containing liquid crystal material andarranged contiguously in a direction of the sequential scan, comprising:a plurality of amplifiers each coupled to drive a respective segment,and a plurality of multiplexors each coupled to a respective amplifierand each selecting one of four voltages to provide to the amplifier inresponse to two control inputs from a controller, said four voltagesbeing a drive voltage of positive polarity, a drive voltage of negativepolarity, a bias voltage of positive polarity and a bias voltage ofnegative polarity, said controller comprising: a circuit receiving ascan signal from the display system, wherein the scan signal is in ahigh state when one image perspective is visible and a low state whenanother image perspective is visible, an oscillating crystal, and acircuit calculating timing and generating said two control inputs basedon the transition of the scan signal between high and low states,wherein a first of the control inputs is set to one state to select adrive voltage and another state to select a bias voltage, and wherein asecond of the control inputs is set to one state to select a positivepolarity and to another state to select a negative polarity.
 13. A drivecircuit as in claim 12 wherein the timing calculating circuit defines afield time as the length of time between transitions of the scan signal,a blanking time as a fraction of the field time and a segment time as afraction of the field time, and wherein the control inputs are generatedto drive each segment in correspondence with the sequential scan foreach image perspective.
 14. A drive circuit as in claim 13, wherein themodulator includes five segments, and wherein the blanking time isdefined as 1/16 of the field time and the segment time is defined as3/16 of the field time, and wherein the control inputs are generated todrive each segment in correspondence with the sequential scan for eachperspective image.
 15. A drive circuit as in claim 14, wherein thecontrol inputs are generated to drive each segment just prior to thecorresponding sequential scan for each perspective image.
 16. A drivecircuit as in claim 15, wherein the control inputs are generated todrive each segment approximately two milliseconds prior to thecorresponding sequential scan for each perspective image.